1. Field of the Invention
The invention relates to devices for gripping tubular members, such as pipes or shafts, so that said members may be rotated axially, or secured against axial rotation. More particularly the invention relates to devices for gripping tubulars, and transmitting substantial torque to the tubular, without use of biting teeth or other hardened gripping surfaces which may cause surface or structural damage to the pipe.
2. Description of Prior Art
In virtually every industrial field there is at least some requirement for gripping tubulars so that they may be axially rotated or secured against rotation. Perhaps the best known requirement for gripping and axially rotating (and securing) tubulars is the assembly and disassembly of threaded connections. Such assembly and disassembly is extremely common in the earth boring industry, particularly that involving oil and gas exploration, where a single well can involve strings of pipe thousands of feet long, which must be assembled and disassembled several times at the surface on a piece-by-piece basis.
Because of the need to repetitively assemble and disassemble threaded connections, particularly in the oil field, various devices, generally referred to as tongs, have been developed to facilitate that task. One tong, generally called the power tong, rotates a first threaded member axially, while another tong, generally called the back up tong, secures a second, mating member against rotation.
Pipe used in the earth boring industry, particularly in the oil and gas field, is frequently subject to severe service. As deeper wells are drilled, the weight of the pipe string increases, as does the internal and external pressures the pipe must bear. The minerals produced are sometimes highly corrosive. Consequently, in some applications, even minor damage to the pipe can lead to premature failure of the pipe in the well bore. Even minor scoring or localized work hardening of the surface of the pipe can cause stress build-ups which can cause the tubular to rupture under internal pressure or crack during assembly or disassembly. Even minor damage to protective coatings can result in premature failure of the tubular due to corrosion. Considering the substantial cost of certain tubular goods, and the cost, time, and danger associated with pipe failure in a well bore, it is highly desirable to avoid all pipe damage during assembly and disassembly of threaded connections.
Various gripping devices, for tubular goods, are known in prior art, but generally these rely on teeth, which bite into the surface of the pipe, to obtain sufficient grip to impart the high torque loads required to create a tight, leak proof connection, for example see U.S. Pat. Nos. 3,545,313, 3,796,418 and 3,912,473.
Mosing U.S. Pat. No. 4,372,026 discloses an improved tong design, utilizing smooth cam surfaces to frictionally grip pipe. While this design represents a significant improvement over use of biting teeth, in very high torque applications, or where relatively soft pipes are used, even the smooth gripping surfaces, applied at discrete, small areas around the pipe's circumference, with high radially inward forces, could cause surface or structural damage to the tubular.
Coyle U.S. Pat. No. 4,712,284 discloses tongs utilizing several discrete, relatively rigid, smooth faced jaws, of relatively soft material to frictionally grip pipe. However, this device does not entirely avoid the underlying problem, which is application of high radially inward forces at discrete areas around the circumference of the pipe. Application of such forces can cause surface or structural damage when high torque or soft pipe is used. Coyle, in fact mentions its preference for serrated, rather than smooth, gripping surfaces when some surface damage to the pipe, such as is said to be the case with fiberglass pipe, can be tolerated. When such serrated jaws are used this device is not truly a pure friction grip device, but rather relies at least partially on biting of the pipe surface to obtain sufficient grip to impart the high torque loads required for sufficient tightening of threaded connection.
These patents do not disclose the present invention. Hitherto, prior art means for gripping tubular members has been by use of teeth, or use of frictional surfaces engaging the pipe in relatively small, discrete areas; or by combination of both means. (Both means have certain disadvantages. Teeth score the pipe and/or pipe coating. Smooth, rigid frictional engagement surfaces in small, discrete areas necessitate the use of high radial contact force per unit of contact area, which can lead to deformation of the cross-sectional area of the pipe and/or localized work hardening of the contact area.
The invention disclosed herein represents a significant improvement over prior art in that it distributes the radial forces, which are required to achieve high torque loads, more uniformly around the tubular's entire circumference and over a substantial axial length of the tubular. By distributing the radial force uniformly over a large contact area the frictional (tangential) forces required for satisfactory tightening of the tubular may be achieved with lowered gripping (radial) forces per unit of contact area. Further, the flexible nature of the gripping surface described herein automatically compensates for minor manufacturing irregularities of the tubular, such as lack of perfect roundness, surface distortions, etc.